There is a need in industrial and scientific processes to cool samples to very low temperatures in order to obtain and/or observe their low temperature properties. This need has arisen in part because of the growing interest in superconductive materials, that is, materials that conduct electricity with minimal resistance to electric current flow. Typically, superconductivity is observed in certain materials that are cooled to 10.degree. K., (-263.degree. C.,) or below. The need to cool samples to extremely low temperatures is also growing because of the increasing desirability to observe and/or obtain other desirable properties, besides superconductivity, that are present in the samples when they are so cooled.
Currently, helium stage refrigerators and cryostats are used to cool samples to very low temperatures. Helium-stage refrigerators employ helium as a refrigerant to drain thermal energy from a sample so as to cool the sample. Typically, helium stage refrigerators operate by providing liquid helium in a closed-loop refrigeration system. The sample is exposed to the liquid helium, which extracts the thermal energy therefrom and converts the energy into the latent heat of evaporation so that the helium evaporates. The evaporation of the liquid helium produces gaseous helium that must be re-condensed, usually by a compression process, so that it can again function as a sink for the sample's thermal energy.
A cryostat includes a sample chamber in which samples are cooled. The cryostat includes a cryogen finger that extends into the sample chamber. A cold fluid, referred to as a cryogen, is circulated through the cryogen finger. The sample is cooled by the transfer of thermal energy from the sample to the cryogen. Heat absorbed by the cryogen causes it to either evaporate or expand so that it is exhausted from the cryogen finger. A recycling system, for example a compressor, is used to restore the cryogen to its original state so that it can again be pumped to the cryogen finger.
There are a number of limitations and disadvantages associated with the low temperature cooling systems currently available. Helium stage refrigerators have low heat exchange capabilities and consequently cool samples at a slow rate. Moreover, compression units, that are an integral part of helium-stage refrigerators, occupy a significant amount of space. There are difficulties associated with isolating samples in helium-stage refrigerators so that they are equally cooled over their entire surface area. Furthermore, the costs of providing the refrigerators, and the required compression units, is costly.
There are also limitations associated with currently available cryostats. Placing a sample in, and removing it from, a cryostat often requires a number of complicated steps. To date, it has been difficult to provide a cryostat that can be used with both liquid-state cryogen and gaseous-state cryogen. In other words, current cryostats typically are designed to be supplied with only a liquid cryogen, or only a gaseous cryogen. The utility of individual cryostats is thus limited because sometimes it is desirable to supply them with cryogen in a state that they cannot accept. These situations occur when there is a need to provide a specific type of cryogen in order to cool a sample to a specific temperature, or through a range of temperature, or when it would be desirable to provide a specific cryogen that requires only minimal auxiliary supply equipment and/or that can be provided with minimal expense.
Also, it is desirable in some instances to cyclically cool and heat a sample through a wide range of temperatures, for example from below 10.degree. K. to above 700.degree. K. (-263.degree. C. to 430.degree. C.). To date, it has provided difficult to provide a system that can be used cool and heat a sample through a cycle of very low temperatures, intermediate temperatures, and very high temperatures, and then repeat or reverse the cycle.
Furthermore, in order to perform some experiments with very cold samples, it is necessary to expose the samples to selected gases while simultaneously cooling or heating both the gas and the sample to a selected temperature that may be either extremely cold or hot. This procedure is performed in order to observe how the gas and the sample react together at a given temperature. Systems that can be used to heat samples and simultaneously expose them to a gas are known. However, to date, it has been very difficult to simultaneously cool a sample to a low temperature, and expose it to a gas.